Diabetes mellitus is a metabolic disorder characterized by recurrent or persistent hyperglycemia (high blood glucose) and other signs, as distinct from a single disease or condition. Glucose level abnormalities can result in serious long-term complications, which include cardiovascular disease, chronic renal failure, retinal damage, nerve damage (of several kinds), microvascular damage and obesity.
Type 1 diabetes, also known as Insulin Dependent Diabetes Mellitus (IDDM), is characterized by loss of the insulin-producing β-cells of the islets of Langerhans of the pancreas leading to a deficiency of insulin. Type-2 diabetes previously known as adult-onset diabetes, maturity-onset diabetes, or Non-Insulin Dependent Diabetes Mellitus (NIDDM)—is due to a combination of increased hepatic glucose output, defective insulin secretion, and insulin resistance or reduced insulin sensitivity (defective responsiveness of tissues to insulin).
Chronic elevation of blood glucose level leads to damage of blood vessels. In diabetes, the resultant problems are grouped under “microvascular disease” (due to damage to small blood vessels) and “macrovascular disease” (due to damage to the arteries). Examples of microvascular disease include diabetic retinopathy, neuropathy and nephropathy, while examples of macrovascular disease include coronary artery disease, stroke, peripheral vascular disease, and diabetic myonecrosis.
Diabetic retinopathy, characterized by the growth of weakened blood vessels in the retina as well as macular edema (swelling of the macula), can lead to severe vision loss or blindness. Retinal damage (from microangiopathy) makes it the most common cause of blindness among non-elderly adults in the US. Diabetic neuropathy is characterized by compromised nerve function in the lower extremities. When combined with damaged blood vessels, diabetic neuropathy can lead to diabetic foot. Other forms of diabetic neuropathy may present as mononeuritis or autonomic neuropathy. Diabetic nephropathy is characterized by damage to the kidney, which can lead to chronic renal failure, eventually requiring dialysis. Diabetes mellitus is the most common cause of adult kidney failure worldwide. A high glycemic diet (i.e., a diet that consists of meals that give high postprandial blood sugar) is known to be one of the causative factors contributing to the development of obesity.
Glucokinase (GK), also known as hexokinase IV or D, is one of four glucose-phosphorylating enzymes called hexokinases that catalyze the first step of glycolysis, the conversion of glucose to glucose 6-phosphate (G6P), in vertebrate tissues. GK functions in a dual role, with distinct functions in the pancreas and liver; (a) as a molecular glucose sensor in the insulin-producing pancreatic β-cells, and (b) as the high-capacity enzymatic step initiating the storage of glucose in the form of glycogen in the liver and uptake of glucose during hyperglycemia. Therefore, GK plays a central role in glucose homeostasis, through the phosphorylation of glucose in the liver, and the modulation of insulin secretion in the pancreas (Postic, C. et al (1999) J. Biol. Chem. 274: 305-315). GK also functions as a sensor in other neuroendocrine cells of the gastrointestinal tract and in various brain cells including specific cells in the hypothalamus (Jetton, T. A. et al (1994) J. Biol. Chem. 269: 3641-3654).
The physiological concentration of glucose in human plasma is approximately 5.5 mM under fasting conditions, and increases to about 12 mM in the fed state. This concentration is dependent on and maintained by the activity of GK, which senses glucose and controls metabolic flux in key cell types. The glucose concentration at which GK activity is at half of its maximal velocity or Vmax is defined as its S0.5. The S0.5 of GK for glucose lies in the middle of the physiological glucose concentration range at approximately 8 mM, allowing this enzyme to act as a molecular glucose sensor crucial for glucose homeostasis. The limited tissue distribution and unique kinetic properties of GK allow it to play a critical role in pancreatic β-cell insulin secretion and hepatic glucose utilization. GK differs from the other members of the mammalian hexokinase family in its unique sigmoidal kinetics with respect to glucose, a high S0.5 that lies in the physiological glucose concentration range (the other three mammalian hexokinases have S0.5 values less than 0.5 mM), the lack of product inhibition by G6P, and its tissue distribution in cell types that are thought to be responsive to changing plasma glucose levels.
Tissue-specific differences have been observed between the regulation of GK in the liver and the pancreas. In the liver, GK is allosterically inhibited by the glucokinase regulatory protein (GKRP), which results in its sequestration in the nucleus and subsequent protection from proteolytic degradation. This inhibition is reversed by high concentrations of glucose and by fructose 1-phosphate, and is potentiated by fructose 6-phosphate. In the pancreatic β-cells, GK expression is believed to be constitutive. GK is also known to be expressed in the hypothalamus, where it may exert effects on feeding behavior, and in the intestine, where it may contribute to the secretion of enteroincretins such as glucagon-like peptide-1 (GLP-1).
Given the role of GK as a molecular glucose sensor, it is not surprising that GK mutations have a profound influence on glucose homeostasis. About 2000 GK mutations that have been identified in humans result in impaired glucose-mediated insulin secretion and maturity-onset diabetes of the young type 2 (MODY-2). Some of these mutations result in decreased accumulation of hepatic glycogen, while others decrease GK activity by reducing the stability of the enzyme or by decreasing its V. Mutations that result in activation of GK are implicated in the onset of persistent hyperinsulinemic hypoglycemia of infancy (PHHI). Single point mutations (e.g. V62M, D158A, Y214A, V455M, and F456V) in regions distinct from the substrate binding site of the enzyme lead to modulation of GK activity (Glaser, B. et al (1998) N. Engl. J. Med. 338: 226-230; Gloyn, A. L. (2003) Hum. Mutat. 22: 353-362; Gloyn, A. L. et al (2003) Diabetes 52: 2433-2440). These observations highlight that GK activity can be regulated through allosteric modulation.
Homozygous knock out of GK in mice results in severe diabetes and death, while heterozygous disruption results in a milder diabetic phenotype, decreased hepatic glucose uptake and impaired insulin secretion in response to glucose. Conversely, overexpression of GK in fat-induced diabetic as well as non-diabetic mice results in improved glucose tolerance. Transgenic mice overexpressing GK in the liver show a modest (20%) increase in fasting GK activity, which correlates with lower fasting plasma glucose and insulin, and improved glucose tolerance (Hariharan, N. et al (1997) Diabetes 46: 11-16).
The enzymatic properties of GK can be described in terms of its velocity (i.e. its rate of converting glucose to G6P) and its S0.5 for glucose (i.e. the apparent glucose concentration at which GK converts glucose to G6P at half of its maximal velocity). The S0.5 of human GK for glucose is approximately 8 mM in enzyme based assay. GKAs induce increased conversion by GK of glucose to G6P by either decreasing the S0.5 of GK for glucose, increasing its Vmax, or by a combination of both, and can potentially lower blood glucose concentrations to hypoglycemic levels.
Several patent applications and publications describe the discovery of small-molecule glucokinase activators (GKAs) that allosterically modulate the activity of GK (Kamata, K. et al (2004) Structure 12: 429-438; WO 2003/055482 A1; WO 2005/123132 A2; WO 2004/002481 A2; U.S. Pat. No. 6,486,184 B2; WO 2006/040528 A1; Fyfe, M. C. T. (2007) Diabetologia, 50: 1277-1287; McKerrecher, D. et al Bioorg. Med. Chem. Lett. 15 (2005) 2103-2106; Efanov, A. M. et al (2005) Endocrinology 146: 3696-3701; Printz, R. L. and Granner, D. K. (2005) Endocrinology 146: 3693-3695; Brocklehurst, K. J. et al (2004) Diabetes, 53: 535-541; Grimsby, J. et al (2003) Science 301: 370-373). These GKAs increase GK activity by decreasing its S0.5 for glucose, and, in some cases, also increasing its Vmax. However, for many of these compounds, hypoglycemia has been reported in animal studies which may be a consequence of excessive GK activation. For example, GK activators like Ro-28-1675 cause hypoglycemia in animal efficacy models (Kamata, K. et al (2004) Structure 12: 429-438). Similar hypoglycemic potential is seen in another GK activator, PSN-GK1, at higher dose (Fyfe, M. C. T. (2007) Diabetologia, 50: 1277-1287).
A concept of minimizing hypoglycemic potential by liver selective glucokinase activation has been mentioned in patent application WO 2005/123132. This concept relied on tissue specific role of glucokinase regulatory protein (GKRP) in liver which is absent in pancreas. A series of small molecule glucokinase activators have also been described as liver selective activators of glucokinase in patent application WO 2004/002481, and these molecules will have less hypoglycemic potential. However, there is no biological data disclosed in this application to support this hypothesis.
The present disclosure provides a novel class of compounds characterized as glucokinase activators and their potential use as medicament for the prophylactic or therapeutic treatment of hyperglycemia, diabetes, obesity, dyslipidemia, metabolic syndrome and like.
This present disclosure also provides a method of identifying compounds that are partial GK activators. Such partial GK activators identified using the said method will be useful for the treatment of hyperglycemia, diabetes, obesity, dyslipidemia, metabolic syndrome and like, in mammals and have minimum hypoglycemic potential.